“Colors at Sunrise and Sunset”

Air Date: Saturday, September 30, 2006

From the STORM Project at the University of Northern Iowa, I’m meteorology professor Alan Czarnetzki. In this week’s installment of “Weather Extra,” we’ll take a look at the reasons behind the colors we commonly see at sunrise and sunset.

Sunrise is occurring later each day this time of year. That means more of us are likely to actually see sunrise along with the beautiful red and orange colors that accompany it. Why is it that the sun has such a red coloration at sunrise and sunset, yet the noon sun appears more yellow? Well, sunshine is white light, a combination of the colors red, orange, yellow, green, blue, indigo, and violet. Each of these colors is a different wavelength of visible light. The color with the shortest wavelength is violet, while red is longest. As all of the wavelengths of visible light are coming into our atmosphere, they encounter obstacles. The individual atoms and molecules in our atmosphere are, in fact, big enough to redirect the incoming sunlight. This redirection by the nitrogen and oxygen and other molecules sends visible light in all directions. This process is known as scattering.

Well, it turns out that not all colors are scattered equally. The small size of the molecules in our atmosphere relative to the wavelengths of visible light results in the preferential scattering of the shortest wavelengths of light…in other words, the blues and violets. This process is known as Rayleigh scattering. If enough of the blue and violet light is scattered out of an incoming beam of sunlight, the combination of the remaining colors will no longer result in white light. The reason we, as kids, all reached for the yellow crayon when drawing the sun is because the net effect of the remaining colors once the blues and violets have been removed…is yellow. However, near sunrise and sunset…when the sun is low in the sky…sunlight has to pass through more of the lower part of the atmosphere before it reaches our eyes than it has to at noon. The lower part of the atmosphere is where air density is greatest. That means that sunlight coming in at sunrise and sunset encounters more obstacles than at noon. As a result, the sunlight is exposed to many more potential scattering events. After the blues and violets have been scattered out, next comes yellow and green. So, as the sun gets lower in the sky it starts to take on more of an orange appearance. Finally, when the sun is near the horizon, even the orange is mostly removed from the sunlight that reaches our eyes and only red is left.